Engines that operate under lean conditions and high compression ratios, including but not limited to diesel engines, provide the benefit of high fuel economy over conventional gasoline engines. Aftertreatment systems for such engines may include a particulate filter for trapping particulate matter in the engine emissions, and also may include a selective catalytic reduction (SCR) system for treating nitrogen oxide (NOx) emissions.
One type of SCR system utilizes a catalyst and an injector for injecting an aqueous urea solution into the exhaust stream upstream of the catalyst. The urea decomposes into ammonia, which acts as a reductant for NOx reduction at the catalyst. Ammonia is generated from urea largely by the following two reaction steps.NH2—CO—NH2(g or l)→NH3(g)+HNCO(g)  (1)HNCO(g)+H2O(g)→NH3(g)+CO2(g)  (2)Reaction (1) is a thermal decomposition reaction, and reaction (2) is a hydrolysis reaction. The thermal decomposition of urea is slow at lower exhaust temperatures, and tends to be the rate limiting step, especially at temperatures below 300° C. Therefore, the spray of urea solution into the diesel exhaust system may result in the accumulation of deposits of urea on the SCR catalyst before the urea decomposes if the exhaust temperature is below 300° C.
To ensure proper particulate filter performance, a particulate filter may be regenerated periodically by increasing exhaust temperatures to 550-650° C. to burn off accumulated particulate matter. Since both the SCR catalyst and the particulate filter are located along the exhaust system, the SCR catalyst may also be heated to the particulate regeneration temperatures during particulate filter regeneration. This may cause urea deposits in the SCR catalyst to decompose at a high rate when the catalyst is heated, which may heat the SCR catalyst to undesirably high temperatures.
Further, unburnt and/or partially burnt hydrocarbons, including but not limited to large size hydrocarbons such as those in diesel fuel, in the exhaust gas may be stored in the SCR catalyst and be oxidized quickly to heat the SCR catalyst to undesirably high temperatures during particulate filter regeneration.
The inventors herein have realized that such issues may be addressed by regenerating the particulate filter by exposing a catalyst and particulate filter to a first, lower elevated exhaust temperature to remove urea deposits and stored hydrocarbon from the catalyst, and then exposing the catalyst and particulate filter to a second, higher elevated exhaust temperature to heat the particulate filter to decompose particulate matter in the particulate filter. In this manner, damage caused to the catalyst due to the decomposition and derived reactions of urea deposits and oxidation of stored hydrocarbon during particulate filter regeneration may be avoided.